Top-to bottom path rendering with opacity testing

ABSTRACT

One embodiment of the present invention sets forth a technique for rendering paths by first generating a stencil buffer indicating pixels of the path that should be covered and then covering the path. The paths may be filled or stroked without tessellating the paths. Path rendering may be accelerated when a graphics processing unit or other processor that is configured to perform operations to generate the stencil buffer and cover the path to fill or stroke the path. When the paths are rendered in a top-to-bottom (front-to-back) order, an opacity stencil may be generated and used to avoid determining path coverage and shading for pixels that are opaque.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority benefit to U.S. provisional patent application titled, “Path Rendering,” filed on May 21, 2010 and having Ser. No. 61/347,359. This related application is also hereby incorporated by reference in its entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention generally relates to graphics processing and more specifically to path rendering by covering the path based on a generated stencil buffer with opacity testing.

2. Description of the Related Art

Path rendering is a style of resolution-independent two-dimensional (2D) rendering, often called “vector graphics,” that is the basis for a number of important rendering standards such as PostScript, Java 2D, Apple's Quartz 2D, OpenVG, PDF, TrueType fonts, OpenType fonts, PostScript fonts, Scalable Vector Graphics (SVG) web format, Microsoft's Silverlight and Adobe Flash for interactive web experiences, Open XML Paper Specification (OpenXPS), drawings in Office file formats including PowerPoint, Adobe Illustrator illustrations, and more.

Path rendering is resolution-independent meaning that a scene is described by paths without regard to the pixel resolution of the framebuffer. This is in contrast to the resolution-dependent nature of so-called bitmapped graphics. Whereas bitmapped images exhibit blurred or pixilated appearance when zoomed or otherwise transformed, scenes specified with path rendering can be rendered at different resolutions or otherwise transformed without blurring the boundaries of filled or stroked paths.

Sometimes the term vector graphics is used to mean path rendering, but path rendering is a more specific approach to computer graphics. While vector graphics could be any computer graphics approach that represents objects (typically 2D) in a resolution-independent way, path rendering is a much more specific rendering model with salient features that include path filling, path stroking, dashing, path masking, compositing, and path segments specified as Bèzier curves.

FIG. 1A is a prior art scene composed of a sequence of paths. In path rendering, a 2D picture or scene such as that shown in FIG. 1A is specified as a sequence of paths. Each path is specified by a sequence of path commands and a corresponding set of scalar coordinates. Path rendering is analogous to how an artist draws with pens and brushes. A path is a collection of sub-paths. Each sub-path (also called a trajectory) is a connected sequence of line segments and/or curved segments. Each sub-path may be closed, meaning the sub-path's start and terminal points are the same location so the stroke forms a loop; alternatively, a sub-path can be open, meaning the sub-path's start and terminal points are distinct.

When rendering a particular path, the path may be filled, stroked, or both. As shown in FIG. 1A, the paths constituting the scene are stroked. When a path is both filled and stroked, typically the stroking operation is done immediately subsequent to the filling operation so the stroking outlines the filled region. Artists tend to use stroking and filling together in this way to help highlight or offset the filled region so typically the stroking is done with a different color than the filling.

FIG. 1B is the sequence of paths shown in FIG. 1A with only filling. Filling is the process of coloring or painting the set of pixels “inside” the closed sub-paths of a path. Filling is similar to the way a child would “color in between the lines” of a coloring book. If a sub-path within a path is not closed when such a sub-path is filled, the standard practice is to force the sub-path closed by connecting its end and start points with an implicit line segment, thereby closing the sub-path, and then filling that resulting closed path.

While the meaning of “inside a path” generally matches the intuitive meaning of this phrase, path rendering formalizes this notion with what is called a fill-rule. The intuitive sense of “inside” is sufficient as long as a closed sub-path does not self-intersect itself. However if a sub-path intersects itself or another sub-path or some sub-paths are fully contained within other sub-paths, what it means to be inside or outside the path needs to be better specified.

Stroking is distinct from filling and is more analogous to tracing or outlining each sub-path in a path as if with a pen or marker. Stroking operates on the perimeter or boundary defined by the path whereas filling operates on the path's interior. Unlike filling, there is no requirement for the sub-paths within a path to be closed for stroking. For example, the curve of a letter “S” could be stroked without having to be closed though the curve of the letter “O” could also be stroked.

FIG. 1C is a prior art scene composed of the sequence of paths from FIG. 1A with the stroking from FIG. 1A and the filling from FIG. 1B. FIG. 10 shows how filling and stroking are typically combined in a path rendering scene for a complete the scene. Both stroking and filling are integral to the scene's appearance.

Traditionally, graphics processing units (GPUs) have included features to accelerate 2D bitmapped graphics and three-dimensional (3D) graphics. In today's systems, nearly all path rendering is performed by a central processing unit (CPU) performing scan-line rendering with no acceleration by a GPU. GPUs do not directly render curved primitives so path rendering primitives such as Bèzier segments and partial elliptical arcs must be approximated by lots of tiny triangles when a GPU is used to render the paths. Constructing the required tessellations of a path that is approximated by many short connected line segments can create a substantial CPU burden. The triangles or other polygons resulting from tessellation are then rendered by the GPU. Because GPUs are so fast at rasterizing triangles, tessellating paths into polygons that can then be rendered by GPUs is an obvious approach to GPU-accelerating path rendering.

Tessellation is a fragile, often quite sequential, process that requires global inspection of the entire path. Tessellation depends on dynamic data structures to sort, search, and otherwise juggle the incremental steps involved in generating a tessellation. Path rendering makes this process considerably harder by permitting curved path segments as well as allowing path segments to self-intersect, form high genus topologies, and be unbounded in size.

A general problem with using a GPU to render paths is unacceptably poor antialiasing quality when compared to standard CPU-based methods. The problem is that GPUs rely on point sampling for rasterization of triangular primitives with only 1 to 8 samples (often 4) per pixel. CPU-based scan-line methods typically rely on 16 or more samples per pixel and can accumulate coverage over horizontal spans.

Animating or editing paths is costly because it requires re-tessellating the entire path since the tessellation is resolution dependent, and in general it is very difficult to prove a local edit to a path will not cause a global change in the tessellation of the path. Furthermore, when curved path segments are present and the scaling of the path with respect to pixel space changes appreciably (zooming in say), the curved path segments may need to be re-subdivided and re-tessellation is likely to be necessary.

Additionally, compositing in path rendering systems typically requires that pixels rasterized by a filled or stroked path are updated once-and-only-once per rasterization of the path. This requirement means non-overlapping tessellations are required. So for example, a cross cannot be tessellated as two overlapping rectangles but rather must be rendered by the outline of the cross, introducing additional vertices and primitives. In particular, this means the sub-paths of a path cannot be processed separately without first determining that no two sub-paths overlap. These requirements, combined with the generally fragile and sequential nature of tessellation algorithms make path tessellation particularly expensive. Because of the expense required in generating tessellations, it is very tempting and pragmatic to cache tessellations. Unfortunately such tessellations are much less compact than the original path representations, particularly when curved path segments are involved. Consequently, a greater amount of data must be stored to cache paths after tessellation compared with storing the paths prior to tessellation. Cached tessellations are also ineffective when paths are animated or rendered just once.

Conventional stroking has been performed by approximating paths into sub-pixel linear segments and then tracing the segments with a circle having a diameter equal to a stroke width. Offset curves are generated at the boundary of the stroked path. These offset curves are typically of much higher degree of complexity compared with the linear segments that are traced to generate the stroked path. Determining whether or not each pixel is inside or outside of a stroked path to generate the stroking is mathematically complex. Identification of the pixels to be stroked is equivalent to identifying pixels that are within half of the stroke width of any point along the path to be stroked. More specifically, the pixels to be stroked are within half of the stroke width measured along a line that is perpendicular to the tangent of the path segment being stroked.

In standard path rendering systems, paths are specified as a sequence of cubic and quadratic (non-rational) Bèzier curve segments, partial elliptical arcs, and line segments. While more mathematically complex path segments representations could be used to specify paths, in practice, existing standards limit themselves to the aforementioned path segment types.

Path filling and stroking use the same underlying path specification. For filling, this means the resulting piece-wise boundaries to be filled may be up to third-order (in the case of cubic Bèzier segments) or rational second-order (in the case of partial elliptical arcs). Filling these curved boundaries of Bèzier curves and arcs is clearly harder than filling the standard polygonal primitives in conventional polygonal 2D or 3D rendering where the boundaries (edges) of the polygonal primitives (usually triangles) are all first-order, being linear segments, and often required to be convex. Filling (and stroking) are also harder than conventional line and convex polygon rasterization because paths are unbounded in their complexity whereas line segments and triangles are defined by just 2 or 3 points respectively. A path may contain just a single path segment or it could contain thousands or more.

The boundaries of stroked paths are actually substantially higher order than the third-order segments. The offset curve of non-rational (second-order) quadratic and (third-order) Bèzier curves are eighth- and tenth-order curves respectively. This high order makes exact determination and evaluation of the resulting offset curves for such Bèzier segments intractable for use in direct rendering. In other words, it is quite unreasonable to try to determine exactly the boundary representation of such offset curves and then simply fill them. For this reason, various techniques have been developed to approximate offset curves with sequences of Bèzier, arc, or line segments. These approximate stroke boundaries may then be filled.

Proper stroking is hard because of the mathematical complexity of the boundary of a path's stroke compared to a path's fill. While approximations to the actual stroke boundary can reduce this complexity, such approximations have associated costs due to inaccuracy and the resulting expansion in the number of primitives that must be both stored and processed to render such approximated strokes. For example, the stroke of a quadratic Bèzier segment can be represented with just the segment's 3 control points (along with the per-path stroke width) whereas an approximation of this stroked boundary with line segments might require dozens or even hundreds of triangles to tessellate approximately the stroked region. Indeed the quality of such tessellations depends on the projection of the curved segment to pixel (or screen) space; this means rendering the same stroked curve at different resolutions would necessitate different tessellations.

Paths are commonly rendered with partially opaque (also known as semi-transparent) shading. The color components (typically red, green, and blue) are augmented with an additional color component widely referred to as the alpha component. Alpha is essentially an opaqueness coefficient ranging from 0% to 100%. Blending is used to combine the coloring of rendered paths with existing color values already in the framebuffer. A scene constructed out of paths is rendered in the order the paths are presented such that earlier paths are lower in the blending order with respect to layer paths. The resulting scene's image is often the result of many blended paths layering upon each other based on the rendering order.

Blending is more expensive than simply writing a color because an existing color sample value must be read from the framebuffer, blending math must be performed with the path's corresponding color value (including alpha), and the result is written to the framebuffer. This read-modify-write pattern slows the rate of rendering. Often blending can be avoided if you know that lower layers of path rendering are fully obscured by subsequent rendered paths above the lower layers. Avoiding rendering, shading, and blending in such situations is advantageous to overall rendering performance and power consumption.

Accordingly, what is needed in the art is an improved system and method for rendering blended paths including filling and stroking of paths.

SUMMARY OF THE INVENTION

One embodiment of the present invention sets forth a technique for rendering paths by first generating a path cover stencil buffer indicating pixels of the path that should be covered and then covering the path. The paths may be filled or stroked without tessellating the paths. Path rendering may be accelerated when a GPU or other processor that is configured to perform operations to generate the path cover stencil buffer and cover the path to fill or stroke the path. When the paths are rendered in a top-to-bottom (front-to-back) order, an opacity stencil buffer may be generated and used to avoid determining path coverage for samples in a path that will not be visible, i.e., are behind another path.

Various embodiments of a method of the invention for rendering paths in top-to-bottom order include receiving a top path including top path stencil geometry and top path covering geometry. Path cover stencil buffer state, indicating samples of the top path to be covered, is generated by rendering the top path geometry. During rendering of the top path covering geometry with stencil testing, surviving pixels that are covered by the top path are determined based on a stencil buffer that includes the path cover stencil buffer state and opacity stencil buffer state. The opacity stencil buffer state is updated for the surviving pixels. The surviving pixels are shaded to produce a rendered image of the top path.

The stencil buffer includes path cover stencil buffer state indicating coverage for each individual path as that path is rendered and opacity stencil buffer state indicating opacity for pixels covered by paths already rendered, when the paths are rendered in top-to-bottom order. The opacity stencil buffer state is used during generation of each path cover stencil buffer to avoid performing computations for path geometry that is obscured by a previously rendered path. Similarly, shading of pixels is avoided by using the opacity stencil buffer state and the path cover stencil buffer state to only shade pixels that are not obscured and that are within the path.

BRIEF DESCRIPTION OF THE DRAWINGS

So that the manner in which the above recited features of the present invention can be understood in detail, a more particular description of the invention, briefly summarized above, may be had by reference to embodiments, some of which are illustrated in the appended drawings. It is to be noted, however, that the appended drawings illustrate only typical embodiments of this invention and are therefore not to be considered limiting of its scope, for the invention may admit to other equally effective embodiments.

FIG. 1A is a prior art scene composed of a sequence of paths;

FIG. 1B is the fill for the prior art scene shown in FIG. 1A;

FIG. 1C is the prior art scene of FIG. 1A with the fill of FIG. 1B and the stroked sequence of paths;

FIG. 2A is a block diagram illustrating a computer system configured to implement one or more aspects of the present invention;

FIG. 2B is a block diagram of a parallel processing subsystem for the computer system of FIG. 2A, according to one embodiment of the present invention;

FIG. 3A is a block diagram of a GPC within one of the PPUs of FIG. 2B, according to one embodiment of the present invention;

FIG. 3B is a block diagram of a partition unit within one of the PPUs of FIG. 2B, according to one embodiment of the present invention;

FIG. 3C is a block diagram of a portion of the SPM of FIG. 3A, according to one embodiment of the present invention;

FIG. 4 is a conceptual diagram of a graphics processing pipeline that one or more of the PPUs of FIG. 2B can be configured to implement, according to one embodiment of the present invention;

FIGS. 5A, 5B, 5C, 5D, 5E, 5F, 5G, and 5H illustrate paths that are rendered in top-to-bottom order using an opacity stencil buffer, according to one embodiment of the invention;

FIG. 6 is a conceptual diagram of a graphics processing pipeline that one or more of the PPUs of FIG. 2B can be configured to implement when performing in top-to-bottom path rendering operations, according to one embodiment of the invention; and

FIG. 7 is a flow diagram of method steps for rendering paths in top-to-bottom order, according to one embodiment of the present invention.

DETAILED DESCRIPTION

In the following description, numerous specific details are set forth to provide a more thorough understanding of the present invention. However, it will be apparent to one of skill in the art that the present invention may be practiced without one or more of these specific details. In other instances, well-known features have not been described in order to avoid obscuring the present invention.

System Overview

FIG. 2A is a block diagram illustrating a computer system 100 configured to implement one or more aspects of the present invention. Computer system 100 includes a central processing unit (CPU) 102 and a system memory 104 communicating via an interconnection path that may include a memory bridge 105. Memory bridge 105, which may be, e.g., a Northbridge chip, is connected via a bus or other communication path 106 (e.g., a HyperTransport link) to an I/O (input/output) bridge 107. I/O bridge 107, which may be, e.g., a Southbridge chip, receives user input from one or more user input devices 108 (e.g., keyboard, mouse) and forwards the input to CPU 102 via path 106 and memory bridge 105. A parallel processing subsystem 112 is coupled to memory bridge 105 via a bus or other communication path 113 (e.g., a PCI Express, Accelerated Graphics Port, or HyperTransport link); in one embodiment parallel processing subsystem 112 is a graphics subsystem that delivers pixels to a display device 110 (e.g., a conventional CRT or LCD based monitor). A system disk 114 is also connected to I/O bridge 107. A switch 116 provides connections between I/O bridge 107 and other components such as a network adapter 118 and various add-in cards 120 and 121. Other components (not explicitly shown), including USB or other port connections, CD drives, DVD drives, film recording devices, and the like, may also be connected to I/O bridge 107. Communication paths interconnecting the various components in FIG. 2A may be implemented using any suitable protocols, such as PCI (Peripheral Component Interconnect), PCI-Express, AGP (Accelerated Graphics Port), HyperTransport, or any other bus or point-to-point communication protocol(s), and connections between different devices may use different protocols as is known in the art.

In one embodiment, the parallel processing subsystem 112 incorporates circuitry optimized for graphics and video processing, including, for example, video output circuitry, and constitutes a graphics processing unit (GPU). In another embodiment, the parallel processing subsystem 112 incorporates circuitry optimized for general purpose processing, while preserving the underlying computational architecture, described in greater detail herein. In yet another embodiment, the parallel processing subsystem 112 may be integrated with one or more other system elements, such as the memory bridge 105, CPU 102, and I/O bridge 107 to form a system on chip (SoC).

It will be appreciated that the system shown herein is illustrative and that variations and modifications are possible. The connection topology, including the number and arrangement of bridges, the number of CPUs 102, and the number of parallel processing subsystems 112, may be modified as desired. For instance, in some embodiments, system memory 104 is connected to CPU 102 directly rather than through a bridge, and other devices communicate with system memory 104 via memory bridge 105 and CPU 102. In other alternative topologies, parallel processing subsystem 112 is connected to I/O bridge 107 or directly to CPU 102, rather than to memory bridge 105. In still other embodiments, I/O bridge 107 and memory bridge 105 might be integrated into a single chip. Large embodiments may include two or more CPUs 102 and two or more parallel processing systems 112. The particular components shown herein are optional; for instance, any number of add-in cards or peripheral devices might be supported. In some embodiments, switch 116 is eliminated, and network adapter 118 and add-in cards 120, 121 connect directly to I/O bridge 107.

FIG. 2B illustrates a parallel processing subsystem 112, according to one embodiment of the present invention. As shown, parallel processing subsystem 112 includes one or more parallel processing units (PPUs) 202, each of which is coupled to a local parallel processing (PP) memory 204. In general, a parallel processing subsystem includes a number U of PPUs, where U≧1. (Herein, multiple instances of like objects are denoted with reference numbers identifying the object and parenthetical numbers identifying the instance where needed.) PPUs 202 and parallel processing memories 204 may be implemented using one or more integrated circuit devices, such as programmable processors, application specific integrated circuits (ASICs), or memory devices, or in any other technically feasible fashion.

Referring again to FIG. 2A, in some embodiments, some or all of PPUs 202 in parallel processing subsystem 112 are graphics processors with rendering pipelines that can be configured to perform various tasks related to generating pixel data from graphics data supplied by CPU 102 and/or system memory 104 via memory bridge 105 and bus 113, interacting with local parallel processing memory 204 (which can be used as graphics memory including, e.g., a conventional frame buffer) to store and update pixel data, delivering pixel data to display device 110, and the like. In some embodiments, parallel processing subsystem 112 may include one or more PPUs 202 that operate as graphics processors and one or more other PPUs 202 that are used for general-purpose computations. The PPUs may be identical or different, and each PPU may have its own dedicated parallel processing memory device(s) or no dedicated parallel processing memory device(s). One or more PPUs 202 may output data to display device 110 or each PPU 202 may output data to one or more display devices 110.

In operation, CPU 102 is the master processor of computer system 100, controlling and coordinating operations of other system components. In particular, CPU 102 issues commands that control the operation of PPUs 202. In some embodiments, CPU 102 writes a stream of commands for each PPU 202 to a pushbuffer (not explicitly shown in either FIG. 2A or FIG. 2B) that may be located in system memory 104, parallel processing memory 204, or another storage location accessible to both CPU 102 and PPU 202. PPU 202 reads the command stream from the pushbuffer and then executes commands asynchronously relative to the operation of CPU 102.

Referring back now to FIG. 2B, each PPU 202 includes an I/O (input/output) unit 205 that communicates with the rest of computer system 100 via communication path 113, which connects to memory bridge 105 (or, in one alternative embodiment, directly to CPU 102). The connection of PPU 202 to the rest of computer system 100 may also be varied. In some embodiments, parallel processing subsystem 112 is implemented as an add-in card that can be inserted into an expansion slot of computer system 100. In other embodiments, a PPU 202 can be integrated on a single chip with a bus bridge, such as memory bridge 105 or I/O bridge 107. In still other embodiments, some or all elements of PPU 202 may be integrated on a single chip with CPU 102.

In one embodiment, communication path 113 is a PCI-EXPRESS link, in which dedicated lanes are allocated to each PPU 202, as is known in the art. Other communication paths may also be used. An I/O unit 205 generates packets (or other signals) for transmission on communication path 113 and also receives all incoming packets (or other signals) from communication path 113, directing the incoming packets to appropriate components of PPU 202. For example, commands related to processing tasks may be directed to a host interface 206, while commands related to memory operations (e.g., reading from or writing to parallel processing memory 204) may be directed to a memory crossbar unit 210. Host interface 206 reads each pushbuffer and outputs the work specified by the pushbuffer to a front end 212.

Each PPU 202 advantageously implements a highly parallel processing architecture. As shown in detail, PPU 202(0) includes a processing cluster array 230 that includes a number C of general processing clusters (GPCs) 208, where C≧1. Each GPC 208 is capable of executing a large number (e.g., hundreds or thousands) of threads concurrently, where each thread is an instance of a program. In various applications, different GPCs 208 may be allocated for processing different types of programs or for performing different types of computations. For example, in a graphics application, a first set of GPCs 208 may be allocated to perform patch tessellation operations and to produce primitive topologies for patches, and a second set of GPCs 208 may be allocated to perform tessellation shading to evaluate patch parameters for the primitive topologies and to determine vertex positions and other per-vertex attributes. The allocation of GPCs 208 may vary dependent on the workload arising for each type of program or computation.

GPCs 208 receive processing tasks to be executed via a work distribution unit 200, which receives commands defining processing tasks from front end unit 212. Processing tasks include indices of data to be processed, e.g., surface (patch) data, primitive data, vertex data, and/or pixel data, as well as state parameters and commands defining how the data is to be processed (e.g., what program is to be executed). Work distribution unit 200 may be configured to fetch the indices corresponding to the tasks, or work distribution unit 200 may receive the indices from front end 212. Front end 212 ensures that GPCs 208 are configured to a valid state before the processing specified by the pushbuffers is initiated.

When PPU 202 is used for graphics processing, for example, the processing workload for each patch is divided into approximately equal sized tasks to enable distribution of the tessellation processing to multiple GPCs 208. A work distribution unit 200 may be configured to produce tasks at a frequency capable of providing tasks to multiple GPCs 208 for processing. By contrast, in conventional systems, processing is typically performed by a single processing engine, while the other processing engines remain idle, waiting for the single processing engine to complete its tasks before beginning their processing tasks. In some embodiments of the present invention, portions of GPCs 208 are configured to perform different types of processing. For example a first portion may be configured to perform vertex shading and topology generation, a second portion may be configured to perform tessellation and geometry shading, and a third portion may be configured to perform pixel shading in pixel space to produce a rendered image. Intermediate data produced by GPCs 208 may be stored in buffers to allow the intermediate data to be transmitted between GPCs 208 for further processing.

Memory interface 214 includes a number D of partition units 215 that are each directly coupled to a portion of parallel processing memory 204, where D≧1. As shown, the number of partition units 215 generally equals the number of DRAM 220. In other embodiments, the number of partition units 215 may not equal the number of memory devices. Persons skilled in the art will appreciate that DRAM 220 may be replaced with other suitable storage devices and can be of generally conventional design. A detailed description is therefore omitted. Render targets, such as frame buffers or texture maps may be stored across DRAMs 220, allowing partition units 215 to write portions of each render target in parallel to efficiently use the available bandwidth of parallel processing memory 204.

Any one of GPCs 208 may process data to be written to any of the DRAMs 220 within parallel processing memory 204. Crossbar unit 210 is configured to route the output of each GPC 208 to the input of any partition unit 215 or to another GPC 208 for further processing. GPCs 208 communicate with memory interface 214 through crossbar unit 210 to read from or write to various external memory devices. In one embodiment, crossbar unit 210 has a connection to memory interface 214 to communicate with I/O unit 205, as well as a connection to local parallel processing memory 204, thereby enabling the processing cores within the different GPCs 208 to communicate with system memory 104 or other memory that is not local to PPU 202. In the embodiment shown in FIG. 2B, crossbar unit 210 is directly connected with I/O unit 205. Crossbar unit 210 may use virtual channels to separate traffic streams between the GPCs 208 and partition units 215.

Again, GPCs 208 can be programmed to execute processing tasks relating to a wide variety of applications, including but not limited to, linear and nonlinear data transforms, filtering of video and/or audio data, modeling operations (e.g., applying laws of physics to determine position, velocity and other attributes of objects), image rendering operations (e.g., tessellation shader, vertex shader, geometry shader, and/or pixel shader programs), and so on. PPUs 202 may transfer data from system memory 104 and/or local parallel processing memories 204 into internal (on-chip) memory, process the data, and write result data back to system memory 104 and/or local parallel processing memories 204, where such data can be accessed by other system components, including CPU 102 or another parallel processing subsystem 112.

A PPU 202 may be provided with any amount of local parallel processing memory 204, including no local memory, and may use local memory and system memory in any combination. For instance, a PPU 202 can be a graphics processor in a unified memory architecture (UMA) embodiment. In such embodiments, little or no dedicated graphics (parallel processing) memory would be provided, and PPU 202 would use system memory exclusively or almost exclusively. In UMA embodiments, a PPU 202 may be integrated into a bridge chip or processor chip or provided as a discrete chip with a high-speed link (e.g., PCI-EXPRESS) connecting the PPU 202 to system memory via a bridge chip or other communication means.

As noted above, any number of PPUs 202 can be included in a parallel processing subsystem 112. For instance, multiple PPUs 202 can be provided on a single add-in card, or multiple add-in cards can be connected to communication path 113, or one or more of PPUs 202 can be integrated into a bridge chip. PPUs 202 in a multi-PPU system may be identical to or different from one another. For instance, different PPUs 202 might have different numbers of processing cores, different amounts of local parallel processing memory, and so on. Where multiple PPUs 202 are present, those PPUs may be operated in parallel to process data at a higher throughput than is possible with a single PPU 202. Systems incorporating one or more PPUs 202 may be implemented in a variety of configurations and form factors, including desktop, laptop, or handheld personal computers, servers, workstations, game consoles, embedded systems, and the like.

Processing Cluster Array Overview

FIG. 3A is a block diagram of a GPC 208 within one of the PPUs 202 of FIG. 2B, according to one embodiment of the present invention. Each GPC 208 may be configured to execute a large number of threads in parallel, where the term “thread” refers to an instance of a particular program executing on a particular set of input data. In some embodiments, single-instruction, multiple-data (SIMD) instruction issue techniques are used to support parallel execution of a large number of threads without providing multiple independent instruction units. In other embodiments, single-instruction, multiple-thread (SIMT) techniques are used to support parallel execution of a large number of generally synchronized threads, using a common instruction unit configured to issue instructions to a set of processing engines within each one of the GPCs 208. Unlike a SIMD execution regime, where all processing engines typically execute identical instructions, SIMT execution allows different threads to more readily follow divergent execution paths through a given thread program. Persons skilled in the art will understand that a SIMD processing regime represents a functional subset of a SIMT processing regime.

Operation of GPC 208 is advantageously controlled via a pipeline manager 305 that distributes processing tasks to streaming multiprocessors (SPMs) 310. Pipeline manager 305 may also be configured to control a work distribution crossbar 330 by specifying destinations for processed data output by SPMs 310.

In one embodiment, each GPC 208 includes a number M of SPMs 310, where M≧1, each SPM 310 configured to process one or more thread groups. Also, each SPM 310 advantageously includes an identical set of functional execution units (e.g., execution units and load-store units—shown as Exec units 302 and LSUs 303 in FIG. 3C) that may be pipelined, allowing a new instruction to be issued before a previous instruction has finished, as is known in the art. Any combination of functional execution units may be provided. In one embodiment, the functional units support a variety of operations including integer and floating point arithmetic (e.g., addition and multiplication), comparison operations, Boolean operations (AND, OR, XOR), bit-shifting, and computation of various algebraic functions (e.g., planar interpolation, trigonometric, exponential, and logarithmic functions, etc.); and the same functional-unit hardware can be leveraged to perform different operations.

The series of instructions transmitted to a particular GPC 208 constitutes a thread, as previously defined herein, and the collection of a certain number of concurrently executing threads across the parallel processing engines (not shown) within an SPM 310 is referred to herein as a “warp” or “thread group.” As used herein, a “thread group” refers to a group of threads concurrently executing the same program on different input data, with one thread of the group being assigned to a different processing engine within an SPM 310. A thread group may include fewer threads than the number of processing engines within the SPM 310, in which case some processing engines will be idle during cycles when that thread group is being processed. A thread group may also include more threads than the number of processing engines within the SPM 310, in which case processing will take place over consecutive clock cycles. Since each SPM 310 can support up to G thread groups concurrently, it follows that up to G*M thread groups can be executing in GPC 208 at any given time.

Additionally, a plurality of related thread groups may be active (in different phases of execution) at the same time within an SPM 310. This collection of thread groups is referred to herein as a “cooperative thread array” (“CTA”) or “thread array.” The size of a particular CTA is equal to m*k, where k is the number of concurrently executing threads in a thread group and is typically an integer multiple of the number of parallel processing engines within the SPM 310, and m is the number of thread groups simultaneously active within the SPM 310. The size of a CTA is generally determined by the programmer and the amount of hardware resources, such as memory or registers, available to the CTA.

Each SPM 310 contains an L1 cache (not shown) or uses space in a corresponding L1 cache outside of the SPM 310 that is used to perform load and store operations. Each SPM 310 also has access to L2 caches within the partition units 215 that are shared among all GPCs 208 and may be used to transfer data between threads. Finally, SPMs 310 also have access to off-chip “global” memory, which can include, e.g., parallel processing memory 204 and/or system memory 104. It is to be understood that any memory external to PPU 202 may be used as global memory. Additionally, an L1.5 cache 335 may be included within the GPC 208, configured to receive and hold data fetched from memory via memory interface 214 requested by SPM 310, including instructions, uniform data, and constant data, and provide the requested data to SPM 310. Embodiments having multiple SPMs 310 in GPC 208 beneficially share common instructions and data cached in L1.5 cache 335.

Each GPC 208 may include a memory management unit (MMU) 328 that is configured to map virtual addresses into physical addresses. In other embodiments, MMU(s) 328 may reside within the memory interface 214. The MMU 328 includes a set of page table entries (PTEs) used to map a virtual address to a physical address of a tile and optionally a cache line index. The MMU 328 may include address translation lookaside buffers (TLB) or caches which may reside within multiprocessor SPM 310 or the L1 cache or GPC 208. The physical address is processed to distribute surface data access locality to allow efficient request interleaving among partition units. The cache line index may be used to determine whether of not a request for a cache line is a hit or miss.

In graphics and computing applications, a GPC 208 may be configured such that each SPM 310 is coupled to a texture unit 315 for performing texture mapping operations, e.g., determining texture sample positions, reading texture data, and filtering the texture data. Texture data is read from an internal texture L1 cache (not shown) or in some embodiments from the L1 cache within SPM 310 and is fetched from an L2 cache, parallel processing memory 204, or system memory 104, as needed. Each SPM 310 outputs processed tasks to work distribution crossbar 330 in order to provide the processed task to another GPC 208 for further processing or to store the processed task in an L2 cache, parallel processing memory 204, or system memory 104 via crossbar unit 210. A preROP (pre-raster operations) 325 is configured to receive data from SPM 310, direct data to ROP units within partition units 215, and perform optimizations for color blending, organize pixel color data, and perform address translations.

It will be appreciated that the core architecture described herein is illustrative and that variations and modifications are possible. Any number of processing units, e.g., SPMs 310 or texture units 315, preROPs 325 may be included within a GPC 208. Further, while only one GPC 208 is shown, a PPU 202 may include any number of GPCs 208 that are advantageously functionally similar to one another so that execution behavior does not depend on which GPC 208 receives a particular processing task. Further, each GPC 208 advantageously operates independently of other GPCs 208 using separate and distinct processing units, L1 caches, and so on.

FIG. 3B is a block diagram of a partition unit 215 within one of the PPUs 202 of FIG. 2B, according to one embodiment of the present invention. As shown, partition unit 215 includes a L2 cache 350, a frame buffer (FB) DRAM interface 355, and a raster operations unit (ROP) 360. L2 cache 350 is a read/write cache that is configured to perform load and store operations received from crossbar unit 210 and ROP 360. Read misses and urgent writeback requests are output by L2 cache 350 to FB DRAM interface 355 for processing. Dirty updates are also sent to FB 355 for opportunistic processing. FB 355 interfaces directly with DRAM 220, outputting read and write requests and receiving data read from DRAM 220.

In graphics applications, ROP 360 is a processing unit that performs raster operations, such as stencil, z test, blending, and the like, and outputs pixel data as processed graphics data for storage in graphics memory. In some embodiments of the present invention, ROP 360 is included within each GPC 208 instead of partition unit 215, and pixel read and write requests are transmitted over crossbar unit 210 instead of pixel fragment data.

The processed graphics data may be displayed on display device 110 or routed for further processing by CPU 102 or by one of the processing entities within parallel processing subsystem 112. Each partition unit 215 includes a ROP 360 in order to distribute processing of the raster operations. In some embodiments, ROP 360 may be configured to compress z or color data that is written to memory and decompress z or color data that is read from memory.

Persons skilled in the art will understand that the architecture described in FIGS. 2A, 2B, 3A, and 3B in no way limits the scope of the present invention and that the techniques taught herein may be implemented on any properly configured processing unit, including, without limitation, one or more CPUs, one or more multi-core CPUs, one or more PPUs 202, one or more GPCs 208, one or more graphics or special purpose processing units, or the like, without departing the scope of the present invention.

In embodiments of the present invention, it is desirable to use PPU 202 or other processor(s) of a computing system to execute general-purpose computations using thread arrays. Each thread in the thread array is assigned a unique thread identifier (“thread ID”) that is accessible to the thread during its execution. The thread ID, which can be defined as a one-dimensional or multi-dimensional numerical value controls various aspects of the thread's processing behavior. For instance, a thread ID may be used to determine which portion of the input data set a thread is to process and/or to determine which portion of an output data set a thread is to produce or write.

A sequence of per-thread instructions may include at least one instruction that defines a cooperative behavior between the representative thread and one or more other threads of the thread array. For example, the sequence of per-thread instructions might include an instruction to suspend execution of operations for the representative thread at a particular point in the sequence until such time as one or more of the other threads reach that particular point, an instruction for the representative thread to store data in a shared memory to which one or more of the other threads have access, an instruction for the representative thread to atomically read and update data stored in a shared memory to which one or more of the other threads have access based on their thread IDs, or the like. The CTA program can also include an instruction to compute an address in the shared memory from which data is to be read, with the address being a function of thread ID. By defining suitable functions and providing synchronization techniques, data can be written to a given location in shared memory by one thread of a CTA and read from that location by a different thread of the same CTA in a predictable manner. Consequently, any desired pattern of data sharing among threads can be supported, and any thread in a CTA can share data with any other thread in the same CTA. The extent, if any, of data sharing among threads of a CTA is determined by the CTA program; thus, it is to be understood that in a particular application that uses CTAs, the threads of a CTA might or might not actually share data with each other, depending on the CTA program, and the terms “CTA” and “thread array” are used synonymously herein.

FIG. 3C is a block diagram of the SPM 310 of FIG. 3A, according to one embodiment of the present invention. The SPM 310 includes an instruction L1 cache 370 that is configured to receive instructions and constants from memory via L1.5 cache 335. A warp scheduler and instruction unit 312 receives instructions and constants from the instruction L1 cache 370 and controls local register file 304 and SPM 310 functional units according to the instructions and constants. The SPM 310 functional units include N exec (execution or processing) units 302 and P load-store units (LSU) 303.

SPM 310 provides on-chip (internal) data storage with different levels of accessibility. Special registers (not shown) are readable but not writeable by LSU 303 and are used to store parameters defining each CTA thread's “position.” In one embodiment, special registers include one register per CTA thread (or per exec unit 302 within SPM 310) that stores a thread ID; each thread ID register is accessible only by a respective one of the exec unit 302. Special registers may also include additional registers, readable by all CTA threads (or by all LSUs 303) that store a CTA identifier, the CTA dimensions, the dimensions of a grid to which the CTA belongs, and an identifier of a grid to which the CTA belongs. Special registers are written during initialization in response to commands received via front end 212 from device driver 103 and do not change during CTA execution.

A parameter memory (not shown) stores runtime parameters (constants) that can be read but not written by any CTA thread (or any LSU 303). In one embodiment, device driver 103 provides parameters to the parameter memory before directing SPM 310 to begin execution of a CTA that uses these parameters. Any CTA thread within any CTA (or any exec unit 302 within SPM 310) can access global memory through a memory interface 214. Portions of global memory may be stored in the L1 cache 320.

Local register file 304 is used by each CTA thread as scratch space; each register is allocated for the exclusive use of one thread, and data in any of local register file 304 is accessible only to the CTA thread to which it is allocated. Local register file 304 can be implemented as a register file that is physically or logically divided into P lanes, each having some number of entries (where each entry might store, e.g., a 32-bit word). One lane is assigned to each of the N exec units 302 and P load-store units LSU 303, and corresponding entries in different lanes can be populated with data for different threads executing the same program to facilitate SIMD execution. Different portions of the lanes can be allocated to different ones of the G concurrent thread groups, so that a given entry in the local register file 304 is accessible only to a particular thread. In one embodiment, certain entries within the local register file 304 are reserved for storing thread identifiers, implementing one of the special registers.

Shared memory 306 is accessible to all CTA threads (within a single CTA); any location in shared memory 306 is accessible to any CTA thread within the same CTA (or to any processing engine within SPM 310). Shared memory 306 can be implemented as a shared register file or shared on-chip cache memory with an interconnect that allows any processing engine to read from or write to any location in the shared memory. In other embodiments, shared state space might map onto a per-CTA region of off-chip memory, and be cached in L1 cache 320. The parameter memory can be implemented as a designated section within the same shared register file or shared cache memory that implements shared memory 306, or as a separate shared register file or on-chip cache memory to which the LSUs 303 have read-only access. In one embodiment, the area that implements the parameter memory is also used to store the CTA ID and grid ID, as well as CTA and grid dimensions, implementing portions of the special registers. Each LSU 303 in SPM 310 is coupled to a unified address mapping unit 352 that converts an address provided for load and store instructions that are specified in a unified memory space into an address in each distinct memory space. Consequently, an instruction may be used to access any of the local, shared, or global memory spaces by specifying an address in the unified memory space.

The L1 Cache 320 in each SPM 310 can be used to cache private per-thread local data and also per-application global data. In some embodiments, the per-CTA shared data may be cached in the L1 cache 320. The LSUs 303 are coupled to a uniform L1 cache 375, the shared memory 306, and the L1 cache 320 via a memory and cache interconnect 380. The uniform L1 cache 375 is configured to receive read-only data and constants from memory via the L1.5 Cache 335.

Graphics Pipeline Architecture

FIG. 4 is a conceptual diagram of a graphics processing pipeline 400, that one or more of the PPUs 202 of FIG. 2 can be configured to implement, according to one embodiment of the present invention. For example, one of the SPMs 310 may be configured to perform the functions of one or more of a vertex processing unit 415, a geometry processing unit 425, and a fragment processing unit 460. The functions of data assembler 410, primitive assembler 420, rasterizer 455, and raster operations unit 465 may also be performed by other processing engines within a GPC 208 and a corresponding partition unit 215. Alternately, graphics processing pipeline 400 may be implemented using dedicated processing units for one or more functions.

Data assembler 410 processing unit collects vertex data for high-order surfaces, primitives, and the like, and outputs the vertex data, including the vertex attributes, to vertex processing unit 415. Vertex processing unit 415 is a programmable execution unit that is configured to execute vertex shader programs, lighting and transforming vertex data as specified by the vertex shader programs. For example, vertex processing unit 415 may be programmed to transform the vertex data from an object-based coordinate representation (object space) to an alternatively based coordinate system such as world space or normalized device coordinates (NDC) space. Vertex processing unit 415 may read data that is stored in L1 cache 320, parallel processing memory 204, or system memory 104 by data assembler 410 for use in processing the vertex data.

Primitive assembler 420 receives vertex attributes from vertex processing unit 415, reading stored vertex attributes, as needed, and constructs graphics primitives for processing by geometry processing unit 425. Graphics primitives include triangles, line segments, points, and the like. Geometry processing unit 425 is a programmable execution unit that is configured to execute geometry shader programs, transforming graphics primitives received from primitive assembler 420 as specified by the geometry shader programs. For example, geometry processing unit 425 may be programmed to subdivide the graphics primitives into one or more new graphics primitives and calculate parameters, such as plane equation coefficients, that are used to rasterize the new graphics primitives.

In some embodiments, geometry processing unit 425 may also add or delete elements in the geometry stream. Geometry processing unit 425 outputs the parameters and vertices specifying new graphics primitives to a viewport scale, cull, and clip unit 450. Geometry processing unit 425 may read data that is stored in parallel processing memory 204 or system memory 104 for use in processing the geometry data. Viewport scale, cull, and clip unit 450 performs clipping, culling, and viewport scaling and outputs processed graphics primitives to a rasterizer 455.

Rasterizer 455 scan converts the new graphics primitives and outputs fragments and coverage data to fragment processing unit 460. Additionally, rasterizer 455 may be configured to perform z culling and other z-based optimizations.

Fragment processing unit 460 is a programmable execution unit that is configured to execute fragment shader programs, transforming fragments received from rasterizer 455, as specified by the fragment shader programs. For example, fragment processing unit 460 may be programmed to perform operations such as perspective correction, texture mapping, shading, blending, and the like, to produce shaded fragments that are output to raster operations unit 465. Fragment processing unit 460 may read data that is stored in parallel processing memory 204 or system memory 104 for use in processing the fragment data. Fragments may be shaded at pixel, sample, or other granularity, depending on the programmed sampling rate.

Raster operations unit 465 is a processing unit that performs raster operations, such as stencil, z test, blending, and the like, and outputs pixel data as processed graphics data for storage in graphics memory. The processed graphics data may be stored in graphics memory, e.g., parallel processing memory 204, and/or system memory 104, for display on display device 110 or for further processing by CPU 102 or parallel processing subsystem 112. In some embodiments of the present invention, raster operations unit 465 is configured to compress z or color data that is written to memory and decompress z or color data that is read from memory.

Generating a Stencil Buffer Indicating Path Coverage

A path consists of zero or more sequences of connected path segment commands for line segments, Bèzier segments, and partial elliptical arcs. A stencil buffer indicating pixel coverage for filling or stroking a path may be generated. The stencil buffer is then used to cover the path, producing a rendered path that is filled and/or stroked. Different techniques are used to prepare the path before the stencil buffer can be generated. When the path will be filled, the path is decomposed into simple cubic Bezier segments and lower order segments. When the path will be stroked, the path is approximated by quadratic Bezier segments and lower order segments. These simple cubic Bezier segments, quadratic Bezier segments, and lower order segments that represent the path and geometry that is rendered to generate the stencil buffer are included in a path specification.

When a path is filled, cubic Bèzier segments pose a particular challenge when these segments are rendered into the stencil buffer to determine which framebuffer sample locations are within the filled region of the respective path. If not done carefully, multiple classes of cubic Bèzier segments can contribute incorrect winding number offsets to the net winding number for a particular framebuffer sample location. An incorrect winding number determination immediately leads to an incorrect determination of the rasterized filled region of said path. Decomposing each arbitrary cubic Bèzier in a path into one or more simple cubic Bèzier segments produces a geometry set that is suitable for rendering filled paths containing cubic Bèzier segments. Such decomposition is beneficial because it results in a robust determination of the filled region of a rendered path without tessellating the path. The path is divided into cubic Bèzier path segments that are each classified and further divided into simple cubic Bèzier path segments. Care must be taken to preserve the proper vertex winding order of each simple Bèzier cubic segment, split the original cubic Bèzier at the proper positions, and linearly interpolate texture.

A cubic Bèzier curve has the freedom, unlike a quadratic Bèzier curve, to specify arbitrary initial and terminal tangent directions for its end-points. This control makes cubic Bèzier curves popular with artists. This additional control comes from the curve being described by a third-order polynomial equation instead of a second-order equation in the case of a quadratic Bèzier curve (and first-order in the case of line segments). This additional polynomial degree provides the requisite freedom for a cubic Bèzier segment to non-trivially self-intersect itself or cross the line formed by the segment's initial and terminal control points. These conditions result in reversals of the local sense of “inside” and “outside” the path. In order for a tessellation-free path filling approach based on stencil counting of rasterized polygons to be robust when a discard shader is used to write a stencil buffer, such situations must be avoided. Additional details of a technique for decomposing a path are described in patent application Ser. No. 13/097,483 filed Apr. 29, 2011, and titled “Decomposing Cubic Bèzier Segments for Tessellation-Free Stencil Filling.”

A set of path cover stencil values in a stencil buffer may be generated that indicates the pixels, or more generally framebuffer sample locations, that are inside of a path to be filled by incrementing each path cover stencil buffer value corresponding to pixels, or more generally samples, that are within front-facing path geometry. Likewise, if the path geometry is back-facing, the rasterization process decrements each path cover stencil value corresponding to pixels within the back-facing path geometry. Path geometry for a path to be filled includes hull geometry and anchor geometry. In another embodiment, stencil values are decremented for front-facing (clockwise winding) primitives and incremented for back-facing (counter-clockwise winding) primitives. Importantly, the decrements, as well as any increments, perform modulo or wrapping arithmetic (rather than saturating arithmetic). This is crucial given the limited integer precision (typically 8 bits) of the stencil buffer. In this example, this means if the stencil buffer was initially cleared to a neutral value, often zero, the result of these decrements to an 8-bit stencil buffer would be the value 255 resulting from modulo-256 arithmetic. By write masking the updates to the stencil buffer, increments and decrements can be computed modulo different powers of two. When one or more bits of each 8-bit stencil value are used to store opacity state, modulo-128 arithmetic can be accomplished using a stencil write mask of 0x7F so that one or more bits (in the most significant bits) are not changed.

In one embodiment, batches of path geometry are drawn together that mix front- and back-facing polygons such that two-sided stencil testing can increment and decrement the stencil based on each polygon's determined facingness. The color and depth writes are disabled during generation of the path cover stencil buffer. Once generation of the path cover stencil values is complete, writes to the color buffer are enabled and the pixels that are inside of the path may be filled by using the stencil buffer to write the color buffer when a conservative bounding geometry referred to as covering geometry, that encloses a closed path is rendered. The covering geometry may be a set of polygons, including a polygon defined by all of the vertices of both the convex hull geometry and anchor geometry for a path to be filled. Alternatively, the covering geometry may be a single polygon that encloses the entire closed path to be filled or stroked. The covering geometry should conservatively enclose the path to be filled or stroked.

When rendering the path geometry and the covering geometry, the vertices belonging to the geometry are subject to an arbitrary projective transformation so the sense of front- or back-facing in object space may be the opposite sense after vertex transformation into pixel space. In one embodiment, the ROP 360 (alternatively raster operations unit 465) performs the increments and decrements of path cover stencil values while the rasterizer 455 rasterizes the path geometry.

As previously explained, a rendered path may be filled and/or stroked. Path stroking has an associated “stroke width” that defines the region that is included in the stroke when a circle having a diameter of the stroke width is moved along the path segment. The path segment is considered a generating curve and the circle generates an inside offset curve and an outside offset curve as the circle moves along the path segment. Mathematical computation of the boundary of such offset curves is difficult. Because stroking is an important operation for many application programs that produce 2D images, it is desirable to accelerate stroking operations. In one embodiment, a GPU, such as the PPU 202, may be used to perform functions to accelerate stroking operations. Importantly, tessellation of the path segments is avoided. Instead, a path is decomposed into quadratic Bèzier path segments or segments of lower complexity, e.g., arcs, line segments, and the like. The stroking operations are accelerated without determining or even approximating the boundary of the strokes (the inside and outside offset curves) that can be defined by high-order polynomials. Instead, computations are performed to determine whether or not discrete point locations are inside or outside of a particular quadratic Bezier stroke or stroke of lower complexity.

Point containment algorithms determine whether a point is “inside” or “outside” the boundary of a closed curve. The process of filling and stroking a path involves determining the set of samples contained within a closed path or the envelope of a path, respectively. Applying some point containment algorithm to each and every sample that is potentially within the boundary defined by the path or stroked boundary is fundamental to the process of stroking a rendered path. Additional details of a technique for point containment during path rendering are described in patent application Ser. No. 13/097,993 filed Apr. 29, 2011, and titled “Point Containment for Quadratic Bèzier Strokes.”

The path geometry for a stroked path includes hull geometry bounding quadratic Bèzier path segments and a set of polygonal geometry for any square or triangular end-caps or mitered or beveled join styles. The stroking engine also collects or generates a set of polygonal geometry for rounded stroking with associated texture coordinates to generate round end-caps, join styles, and hemi-circles for cusps of curved segments converted to line segments. This geometry may include texture coordinates indicating vertex position relative to the junction, end-point, or cusp.

When a path is stroked, the path cover stencil values indicate pixels that are within the stroke region of the path, and the pixels that are inside of the path may be stroked by using the stencil buffer to write the color buffer when covering geometry that encloses a closed path is rendered. During this second rendering pass to cover the path, the path cover stencil values may be cleared to a neutral stencil value for each pixel as a fill color is written to the color buffer for the respective pixel. Clearing the stencil value of each pixel is straightforward to accomplish with standard stencil operations such as Zero or Replace.

Additional details of a technique for performing two-step path rendering are described in patent application Ser. No. 13/100,938 filed May 4, 2011, and titled “Path Rendering by Covering the Path Based on a Generated Stencil Buffer.”

Top-to-Bottom Path Rendering

Conventional path rendering draws paths in bottom-to-top render order. This is inefficient because it often means a pixel is within a path and so colored, but then subsequently completely overlapped by a later path. This means the path renderer spends effort determining the pixel is covered by the path and then coloring the pixel that is essentially discarded. Alternatively, the layers of paths could be rendered in top-to-bottom order such that once a pixel was written with a fully opaque color, then any further processing of that pixel due to lower layers could be skipped. Rather than processing the path layers in the bottom-to-top order like layers of paint applied to a canvas, this top-to-bottom approach reverses the conventional approach. For this reason, this approach is sometimes called Reverse Painter's algorithm.

While processing paths in this “reverse” top-to-bottom order can be more efficient, it requires additional state be maintained with every pixel. In particular, each pixel needs some state to indicate the pixel is “done” being updated. This also means that every time a pixel is about to be processed, this additional “done” state must be checked. And each pixel's “done” state must be set to not-done before starting to render the scene. The “done” state may be thought of as a write-control bit since when a pixel is “done” writes to the color and/or z buffer are effectively disabled.

In the presence of semi-transparent coloring of paths, additional state is necessary to maintain the cumulative opacity at each pixel. And compositing must properly combine this cumulative opacity with the opacity of subsequent layer in the top-to-bottom order. Because of the burdens described in maintaining and consulting this additional state, most CPU-based path renderers do not implement top-to-bottom ordering.

With the two-step “stencil, then cover” approach to path rendering that first generates the path cover stencil values and then covers pixels based on the path cover stencil values, these burdens are not nearly so onerous. GPUs already maintain colors in the framebuffer as RGBA values where the fourth A or alpha (α) component can store opacity information. And one bit of the stencil buffer (typically the most-significant bit) can store whether or not a sample is “done” or not. Checking this “done” bit can be done essentially “for free” by the per-sample stencil test in the graphics processing pipeline 400. Finally, graphics processors are optimized to efficiently discard blocks of samples when an entire group or tile of samples will all fail the stencil test together.

Whereas the Reverse Painter's algorithm typically make CPU path renderers too complicated to justify its implementation, a carefully constructed configuration of the stencil buffer and the use of alpha allows the Reverse Painter's algorithm to be used by the graphics processing pipeline 400 to improve overall path rendering performance. In addition to the path cover stencil values, the stencil buffer also includes opacity state that may be a single bit that functions as the “done” bit. The collection of opacity state stored in the stencil buffer is referred to as the opacity stencil buffer state and the collection of path cover stencil values stored in the stencil buffer is referred to as the path cover stencil buffer state. Only a single bit of stencil per sample position is required to maintain the opacity stencil buffer state. Multiple bits are typically required to maintain the path cover stencil state because they represent, particularly in the case of path filling, a winding number of the sample position with respective to a path's coverage. Standard stencil buffer functionality supports read and write masking so the available stencil value can be segregated into distinct opacity state and coverage state by manipulating the read and write stencil masks.

FIG. 5A illustrates paths that are rendered in top-to-bottom order using an opacity stencil buffer state and a path cover stencil buffer state, according to one embodiment of the invention. A bottom layer 501 includes clouds that are stroked and filled with an opaque color. A middle layer 502 includes a crescent moon that is filled with an opaque color. A top layer 503 includes two starts that are stroked with an opaque color and filled with a non-opaque color. When the paths are rendered in top-to-bottom order, the top layer 503 is rendered first, followed by the middle layer 502, and ending with the bottom layer 501 to produce the image illustrated in FIG. 5A.

FIG. 5B illustrates the path cover stencil buffer, path fill stencil buffer state 505, that is generated when the path geometry for filling paths in the top layer 503 are rendered to generate the stencil buffer, according to one embodiment of the invention. First initialize the entire stencil buffer to 0x0 before rendering any of the paths so that the path cover stencil value is a neutral value (zero) and the opacity stencil value is 0 (false). When generating the path cover stencil buffer by rendering the path geometry, the stencil test function should be GL_NOTEQUAL,0x80,0x80 so that the path cover stencil buffer value is not updated when the opacity stencil value is set. The parameters GL_NOTEQUAL,0x80,0x80 correspond to the stencil function, stencil reference value, and stencil read mask, respectively, as would be passed to OpenGL's glStencilFunc command. In one embodiment, the particular stencil test function state used for path stenciling is set with the command shown in TABLE 1. The glPathStencilFuncNV command operates identically to the standard glStencilFunc command but specifies the particular stencil test function used for path stenciling, ignoring the conventional stencil function test state set by glStencilFunc.

TABLE 1 glPathStencilFuncNV(GL_NOTEQUAL, 0x80, 0x80);

As shown in FIG. 5B, the path fill stencil buffer state 505 samples in the stencil buffer are written for samples that are inside of the two stars in the top layer 503. When the covering geometry for filling the top layer 503 is rendered, corresponding samples in the color buffer will be written with the fill color based on the path fill stencil buffer state 505. The stencil test function during the “cover” path rendering step should be GL_NOTEQUAL,0x80,0x7F (configured as shown in TABLE 2) so that color samples are written with the fill color when the sample's stencil value masked with 0x7F is not zero. The mask 0x7F causes the stencil test to ignore the opacity state, but the earlier stencil testing during the stencil step would not have disturbed these lower bits of the stencil buffer as updates to stencil samples with the opaque state set would be discarded In FIG. 5B white is used to represent 0x0 and black is used to represent values not 0x0 when masked with 0x7F, i.e., samples that will be filled.

TABLE 2 glPathStencilFuncNV(GL_NOTEQUAL, 0x80, 0x7F);

Additionally, the opacity stencil buffer is updated based on the alpha value of the accumulated alpha for the sample. When the accumulated alpha for a sample is fully opaque (typically alpha=0xFF), the opacity stencil value for the sample is set to indicate that the sample is “done.” The stencil value (opacity stencil value and the path cover stencil value) should be 0xFF after the stencil value is updated when the accumulated alpha is opaque. The stencil value should be 0x0 (when masked with 0x7F) after the stencil value is updated when the accumulated alpha is partially opaque. A GL_REPLACE stencil operation will write the stencil value to 0x80 based on the accumulated alpha for each sample. Because the fill color for the top layer 503 is not opaque, the accumulated alpha values resulting from rendering the covering geometry to fill the paths in the top layer 503 are not opaque. Thus, the opacity stencil buffer state corresponding to the path fill stencil buffer state 505 is clear, i.e., 0x0 for all samples.

When non-opaque paths are rendered the path colors are blended with an “under” compositing rule (rather than the standard “over” composite rule, often implemented with SrcAlpha,OneMinusSrcAlpha blending). The “under” rule requires maintaining a per-color sample alpha value (sometimes called “destination alpha”) for blending and using pre-multiplied alpha for colors. Such blending is typically configured as OneMinusDstAlpha,One blending. Destination alpha typically precludes blend optimizations. Use of “Under” blending assumes the color buffer is initially cleared to all zeroes, including the destination alpha component. “Under” blending also requires a final window-sized blend when the background color is exposed and a color other than black.

FIG. 5C illustrates the path cover stencil buffer state, path stroke stencil buffer state 508, that is generated when the path geometry for stroking paths in the top layer 503 are rendered to generate the stencil buffer, according to one embodiment of the invention. As shown in FIG. 5C, samples of the path stroke stencil buffer state 508 are written that are within the path stroke region of the two stars in the top layer 503. When the covering geometry for filling the top layer 503 is rendered, corresponding samples in the color buffer will be written with the fill color based on the path stroke stencil buffer state 508. The stencil test during the “cover” path rendering step should be GL_NOTEQUAL,0x80,0x7F so that color samples are written with the fill color when the sample's stencil value masked with 0x7F is not zero. In other words, samples having a stencil value of zero for the lower 7 bits of stencil buffer are stroked.

FIG. 5D illustrates an opacity stencil buffer state 510 that is generated when the covering geometry for stroking paths in the top layer 503 is rendered to stroke the paths in the top layer 503, according to one embodiment of the invention. When the covering geometry for stroking paths in the top layer 503 is rendered, the opacity stencil buffer is updated based on the alpha value of the accumulated alpha for the sample. Because the stroke color for the top layer 503 is opaque, the accumulated alpha values resulting from rendering the covering geometry to stroke the paths in the top layer 503 are opaque. Thus, the opacity stencil buffer state 510 corresponding to the path fill stencil buffer state 508 is black, e.g., 0x0 when masked with 0x80 in FIG. 5D, except for the stroke region of the top layer 503, where the opacity values are 0x80 or white in FIG. 5D.

FIG. 5E illustrates the path cover stencil buffer state, path fill stencil buffer state 512, that is generated when the path geometry for filling paths in the middle layer 502 are rendered to generate the stencil buffer, according to one embodiment of the invention. As shown in FIG. 5E, samples of the path fill stencil buffer state 512 are written that are within the path fill region of the crescent moon in the middle layer 502. However, the samples that correspond to samples the opacity stencil buffer 510 that are “done” are not written to generate the path fill stencil buffer state 512.

When the covering geometry for filling the middle layer 502 is rendered, corresponding samples in the color buffer will be written with the fill color based on the path fill stencil buffer state 512. The stencil test during the “cover” path rendering step should be GL_NOTEQUAL,0x80,0x7F so that color samples are written with the fill color when 0x0 is not equal to the sample's stencil value masked with 0x7F. In other words, only samples having a stencil value not zero when masked with 0x7F are filled.

FIG. 5F illustrates an opacity stencil buffer state 515 that is generated when the covering geometry for filling paths in the middle layer 502 is rendered to fill the paths in the middle layer 502, according to one embodiment of the invention. When the covering geometry for filling paths in the middle layer 502 is rendered, the opacity stencil buffer is updated based on the alpha value of the accumulated alpha for the sample to generate the opacity stencil buffer state 515. Because the fill color for the middle layer 502 is opaque, the accumulated alpha values resulting from rendering the covering geometry to fill the paths in the middle layer 502 are opaque. Thus, the opacity stencil buffer state 515 corresponding to the path fill stencil buffer state 512 is set, e.g., 0x80 or white in FIG. 5D where the middle layer 502 is filled and where the top layer 503 is stroked in FIG. 5F.

FIG. 5G illustrates the path cover stencil buffer state, path fill stencil buffer state 517, that is generated when the path geometry for filling paths in the bottom layer 501 are rendered to generate the stencil buffer, according to one embodiment of the invention. As shown in FIG. 5G, samples of the path fill stencil buffer state 517 are written that are within the path fill region of the clouds in the bottom layer 501. However, the samples that are inside of the crescent moon and that correspond to samples the opacity stencil buffer state 515 that are “done” are not written to generate the path fill stencil buffer state 517.

When the covering geometry for filling the bottom layer 501 is rendered, corresponding samples in the color buffer will be written with the fill color for the bottom layer 501 based on the path fill stencil buffer state 517. The stencil test during the “cover” path rendering step should be GL_NOTEQUAL,0x80,0x7F so that color samples are written with the fill color when zero is not equal the sample's stencil value masked with 0x7F.

FIG. 5H illustrates an opacity stencil buffer state 520 that is generated when the covering geometry for filling paths in the bottom layer 501 is rendered to fill the paths in the bottom layer 501, according to one embodiment of the invention. When the covering geometry for filling paths in the bottom layer 501 is rendered, the opacity stencil buffer is updated based on the alpha value of the accumulated alpha for the sample to generate the opacity stencil buffer state 520. Because the stroke color for the bottom layer 501 is opaque, the accumulated alpha values resulting from rendering the covering geometry to fill the paths in the bottom layer 501 are opaque. Thus, the opacity stencil buffer state 520 corresponding to the path fill stencil buffer state 517 is clear, e.g., 0x80 or white in FIG. 5G where the bottom and middle layers 501 and 502 are filled and where the top layer 503 is stroked.

Rendering the paths of the bottom layer 501, middle layer 502, and top layer 503 in bottom-to-top order does not require use of an opacity stencil buffer or alpha channel. However, all samples of the paths are rendered to generate the path cover stencil buffer and all samples of the paths are shaded, even those samples that be not be visible in the final image. In contrast, rendering the paths of the bottom layer 501, middle layer 502, and top layer 503 in top-to-bottom requires maintaining the opacity state and maintaining an alpha for each sample in the frame buffer. However, advantageously, only samples that will be visible in the final image are shaded and rendered to generate the path cover stencil buffer. One disadvantage of the described approach is only 7 bits of stencil counting precision are available for determining coverage of filled paths. However, this counting precision is typically sufficient for rendering most path rendered scenes.

Two-Step Path Rendering

In most 3D graphics rendering systems, determining what pixels are contained within a rendered primitive (comparable to path stenciling) and shading those pixels (comparable to path covering) are performed in a single rendering pass. Coupling the stenciling step with the covering step makes sense when the operations require determining what samples are covered by a primitive are bounded and inexpensive. For example, determining the coverage of a triangle requires evaluating exactly three edge equations and determining the sample position satisfies all three inequalities. In contrast, filling a path primitive involves an unbounded number of path segments to consider. Path stroking also requires processing path primitives with an unbounded number of segments, but in addition, the number of operations to determine point containment even of a simple quadratic Bèzier segment is approximately two orders of magnitude more than required for point containment by a triangle.

Path rendering systems also assume that blending (used for both opacity and coverage) is performed once-and-only-once for a covered sample. At least one bit of state per sample (the path cover stencil value) is required to track whether or not a primitive has updated that sample. Additionally, when filling or stroking a path, point containment should be performed per-sample while shading can typically be performed at the per-pixel rate. The stencil test is a per-sample test. Whereas programmable per-fragment color shading typically (meaning most efficiently) runs per-pixel. Acceptable path rendering quality typically requires more than one point containment test per pixel. While the point containment determination may then be performed many times per pixel, it is advantageous to limit the shading computations in the cover step to one shading computation to pixel, particularly when complex shading computations are involved. Actual performance results from two-step GPU-accelerated path rendering, with the stencil generation separated from the path covering, indicate that the two-step technique is substantially faster than other known path rendering systems.

By decoupling the stencil step for path point containment from the cover step for path shading, the cover step can apply an arbitrary shading computation implemented by a standard programmable shader implemented in a standard GPU shading language such as Cg, GLSL, or HLSL or fixed-function operations. This is advantageous because these shaders for covering are no different from the shaders used for conventional 3D shaded rendering. In existing path rendering systems, shading of paths is typically limited to fixed operations that are distinct and different from the programmable shading allowed for 3D rendering. Existing shaders and their extensive systems for compilation and specification can be advantageously used for path rendering as a result of this two-step approach that separates generation of the stencil buffer from covering of the path.

Importantly, the two-step technique takes advantage of the many efficient and parallel units within the GPU. In practice, this means taking advantage of the following performance features of modern GPUs. Some of these performance features include double-rate depth-stencil-only rendering, per-sample shading, bandwidth-efficient stencil operations, two-sided stencil testing, centroid or per-sample interpolation, early (pre-shading) depth-stencil testing, per-tile depth-stencil culling, and the like. Modern GPUs also pipeline multiple render batches to execute them in parallel so multiple stencil and cover steps can be operating within the logical graphics processing pipeline 400 simultaneously while preserving the effect of sequential rendering of the paths.

Before the path cover stencil buffer state is generated, path geometry is generated to fill or stroke the path. The path geometry is then input to a graphics processing pipeline, such as the graphics processing pipeline 400. In one embodiment, the path is broken down into path segments, cubic Bezier path segments are analyzed, simple cubic Bezier path segments are generated, and anchor geometry and convex hull is generated for the path to be filled. Similarly, the path is broken down into path segments, higher-order segments are approximated by quadratic Bezier segments, and conservative bounding hull, end-cap, and join geometry is generated for the path to be stroked. The generated geometry is referred to as path geometry. In addition to the path geometry, the covering geometry is also generated.

FIG. 6 is a conceptual diagram of a graphics processing pipeline that one or more of the PPUs 202 of FIG. 2B can be configured to implement when performing in top-to-bottom path rendering operations, according to one embodiment of the invention. The path and cover geometry for filling or stroking a sequence of rendered paths is input to a vertex and geometry processing units 605 in a top-to-bottom order.

As previously explained, the path geometry is resolution-independent meaning that the filled or stroked path can be rasterized under arbitrary projective transformations without needing to revisit the construction of the path geometry. This resolution-independent property is unlike geometry sets built through a process of tessellating curved regions into line segments; in such circumstances, sufficient magnification of the filled path would reveal the tessellated underlying nature of such a tessellated geometry set. Additionally, the path geometry is compact meaning that the number of bytes required to represent the filled or stroked path is linear with the number of path segments in the original path. This property does not generally hold for tessellated versions of filled paths where the process of subdividing curved edges and introducing tessellated triangles typically bloats out the resulting geometry set considerably.

The vertex and geometry processing units 605 may include one or more of the data assembler 410, the vertex processing unit 415, the primitive assembler 420, the geometry processing unit 425, and the viewport scale, cull, and clip unit 450 shown in FIG. 4. The vertex and geometry processing units 605 may be configured to transform the path geometry (anchor and convex hull geometry for filling and end-cap, join, and conservative bounding hull geometry for stroking) into pixel space.

The 2D (x,y) position of each vertex in the path geometry set can be treated as a 3D (x,y,0) position (using zero for the z component) such that a linearly varying depth values may be generated after projective transformation, and then rasterized to produce per-sample depth values suitable for conventional depth testing during the stencil step, however only the stencil buffer (not the depth buffer) is written during the stencil step so all the depth test can do is discard the stencil update. Similarly depth values can be generated from the covering geometry. Covering geometry depth values can update the depth value when indicated by the prior step's stencil results, subject to depth testing. When depth testing paths in this manner, it is advantageous to apply a polygon offset to the depth values in the stencil step in order to disambiguate the depth values from previously rendered co-planar paths. This approach allows path rendering to mix advantageously with conventional 3D rendering in a manner unavailable to prior art for path rendering.

The rasterizer 455 performs coarse and fine rasterization to determine the sample coverage of the path geometry (pixel or sub-pixel samples). The rasterizer includes determination, when filling, of whether the primitive being rasterized is front- or back-facing. This determination may be used subsequently to determine whether to increment or decrement stencil values. When geometry for convex hull geometry and round end-caps is rasterized, the fragment processing unit 460 is configured to execute the discard shader to determine whether the rasterized fragments should be discarded. If a fragment is discarded, no further processing occurs; in particular, the fragment's stencil value is not disturbed. Geometry for anchor triangles, non-round join styles, and non-round end caps does not require a discard shader.

Implementations may bypass the shader execution for such these primitives to improve rasterization performance and/or reduce overall power consumption. The raster operations unit 465 performs stencil testing and, if configured, depth testing. No further processing or buffer update occurs for fragments that are discarded by the fragment processing unit 460, or which fail the stencil test, or which fail the depth test (if enabled). Deciding the stencil test may require reading stencil buffer 600; deciding the stencil test may require reading the depth buffer 615. If a fragment is not otherwise discarded, raster operations unit 465 performs stencil updates.

To fill the path, the stencil values corresponding to samples in the stencil buffer may be incremented and decremented based on the front- or back-facing winding direction of the path geometry determined by rasterizer 455. Alternatively to fill the path, the stencil values corresponding to the samples in the stencil buffer may be inverted. To stroke the path, the values corresponding to samples in the stencil buffer are set to a designated stencil reference value when the sample is within the stroke region. The raster operations unit 465 writes the stencil buffer 600. Conventional operations to control rasterization are available during generation of the stencil buffer, e.g., clip planes, scissoring, depth offset, window ownership testing, and the like. Importantly, the stencil testing read and write mask allows the coverage stencil buffer state to be segregated from the opacity stencil buffer state.

Covering geometry (geometry fully covering the path) is input to cover the path based on the stencil buffer. The vertex and geometry processing units 605 may be configured to transform the covering geometry into pixel space. Importantly, the same transform that was applied to the path geometry to generate the stencil buffer is applied to the covering geometry to cover the path. As described previously and as done for the stencil step, the covering geometry is transformed and rasterized to produce per-sample depth values that can be used for depth testing and to update the depth buffer.

The rasterizer 455 performs coarse and fine rasterization to determine the sample coverage of the covering geometry (pixel or sub-pixel samples). The raster operations unit 465 is configured to perform stencil operations logically subsequent to shading operations (performed by the fragment processing unit 460). When covering the path, the raster operations unit 465 is configured to read the stencil buffer and use a stencil test to discard samples based on the opacity stencil buffer and the path cover stencil buffer. Samples behind opaque samples or that are not inside of the path when filling or inside of the stroke region when stroking are discarded. When filling a path, rasterized samples that are behind opaque samples or have a corresponding stencil value in the stencil buffer 600 that is equal to the neutral value (value that is typically used to clear the path stencil buffer) are discarded by the stencil test performed by the raster operations unit 465. The neutral value is written to the path cover portion of the stencil buffer 600 after the corresponding stencil value is read. The opacity stencil buffer state is updated to indicate samples that are opaque so that those opaque samples will not be written when paths behind the currently rendered paths are rendered.

After all of the covering geometry has been stencil tested, all of the path cover values in the stencil buffer 600 have been reset to their neutral value and the opacity values for newly opaque pixels are updated. When the two-step technique is used to render of a sequence of paths, the stencil buffer is updated as the covering geometry is rendered. Resetting every sample in the path cover stencil buffer state as a separate operation can be expensive, so performing the reset as part of the path covering operation is advantageous.

The fragments be filled or stroked are shared by the fragment processing unit 460. Standard path rendering modes are available during covering of the path, e.g., constant color, linear gradients, radial gradients, and the like. Advantageously, the fragment processing unit 460 can perform arbitrary programmable shading computations, matching those available to 3D rendering such as texturing, bump mapping, noise generation, and shadow mapping. The raster operations unit 465 writes the image buffer 610 and may also be configured to write depth values computed by the rasterizer 455 and/or fragment processing unit 460 to a depth buffer 615 and perform depth testing. When consistent with the order of operations described, an implementation may advantageously perform the stencil and/or depth tests normally performed by the raster operations unit 465 prior to shading operations performed by fragment processing unit 460. This reordering is not possible if the fragment shader might discard fragments or compute an alternative depth value to be used for depth testing. Conventional operations to control rasterization are available while rendering the covering geometry, e.g., clip planes, scissoring, depth offset, window ownership testing, and the like.

FIG. 7 is a flow diagram of method steps for rendering paths in top-to-bottom order, according to one embodiment of the present invention. Although the method steps are described in conjunction with the systems of FIGS. 2A, 2B, 3A, 3B, 3C, 4, and 6, persons skilled in the art will understand that any system configured to perform the method steps, in any order, is within the scope of the invention. The CPU 102 or parallel processing subsystem 112 may be configured to render paths.

At step 705 a path object for a path in the top layer is received. At step 710 the stencil buffer is initialized to clear the opacity values (indicate that all samples are non-opaque) and the path cover values. The stencil buffer is also configured for stencil testing to discard samples based on the opacity stencil values. At step 715 the specification of the path including the path geometry is provided to one or more of the PPUs 202 or another processor and the path geometry are rendered to generate a stencil buffer 600 indicating samples of the path to be covered. More specifically, the path cover stencil values in the stencil buffer are computed.

At step 720 the stencil testing is configured to discard samples based on the stencil buffer (opacity stencil values and path cover stencil values). At step 725 the covering geometry is rendered with stencil testing enabled to determine surviving pixels that are covered by the path based on the stencil buffer 600. The stencil testing will discard any pixels that are “done” based on the opacity stencil buffer values. Pixels that are not “done” will be discarded based on the path cover stencil values to generate the surviving pixels. At step 730 the surviving pixels are shaded to cover the path, producing a rendered image of the path. The rendered image may illustrate the path with stroking and/or filling. At step 730 the stencil buffer is also updated based on the accumulated opacity for each pixel that is covered according to the stencil buffer. More specifically, when the accumulated opacity for a pixel indicates that that the color is opaque, the opacity value for the pixel is set in the stencil buffer. The path cover value for each pixel that is covered according to the stencil buffer is cleared in order to generate a path cover stencil buffer for the next lower (further behind) path.

At step 735, the shader program determines if another path will be rendered, and, if not, at step 740 the path rendering is done. Otherwise, at step 745, the stencil testing is configured to discard samples based on the opacity stencil buffer for generation of the path cover stencil buffer.

The two-step path rendering technique of “stencil, then cover” path rendering turns the path rendering problem into a rasterization problem at which GPUs are exceedingly efficient. The stencil buffer is used to maintain opacity state in addition to path coverage information to avoid rendering samples that are not visible when the paths are rendered in top-to-bottom order.

One embodiment of the invention may be implemented as a program product for use with a computer system. The program(s) of the program product define functions of the embodiments (including the methods described herein) and can be contained on a variety of computer-readable storage media. Illustrative computer-readable storage media include, but are not limited to: (i) non-writable storage media (e.g., read-only memory devices within a computer such as CD-ROM disks readable by a CD-ROM drive, flash memory, ROM chips or any type of solid-state non-volatile semiconductor memory) on which information is permanently stored; and (ii) writable storage media (e.g., floppy disks within a diskette drive or hard-disk drive or any type of solid-state random-access semiconductor memory) on which alterable information is stored.

The invention has been described above with reference to specific embodiments. Persons skilled in the art, however, will understand that various modifications and changes may be made thereto without departing from the broader spirit and scope of the invention as set forth in the appended claims. The foregoing description and drawings are, accordingly, to be regarded in an illustrative rather than a restrictive sense. 

The invention claimed is:
 1. A method of rendering paths in top-to-bottom order, the method comprising: receiving a top path including top path geometry and top path covering geometry; generating, by rendering the top path geometry, path cover stencil buffer state indicating samples of the top path to be covered; determining, during rendering the top path covering geometry with stencil testing, surviving pixels that are covered by the top path based on a stencil buffer that includes the path cover stencil buffer state and opacity stencil buffer state; updating the opacity stencil buffer state for the surviving pixels; and shading the surviving pixels to produce a rendered image of the top path.
 2. The method of claim 1, further comprising: receiving a next path that is below the top path and includes next path geometry and next path covering geometry; generating, by rendering the path geometry of the next path, a next path cover stencil buffer indicating samples of the next path to be covered limited by the opacity stencil buffer state.
 3. The method of claim 2, further comprising: determining, during rendering the next path covering geometry with stencil testing, surviving pixels that are covered by the next path based on the stencil buffer that includes the next path cover stencil buffer and opacity stencil buffer state; updating the opacity stencil buffer state for the surviving pixels; and shading the surviving pixels to produce a rendered image of the top path and the next path.
 4. The method of claim 2, wherein the generating of the next path cover stencil buffer includes discarding samples having corresponding stencil values in the opacity stencil buffer state indicating that respective samples of the rendered image are opaque.
 5. The method of claim 1, further comprising updating the opacity stencil buffer state for each surviving pixel that is opaque after shading each respective surviving pixel.
 6. The method of claim 1, wherein the opacity stencil buffer state comprises a most significant bit stored for each sample in the stencil buffer.
 7. The method of claim 1, wherein determining the surviving pixels includes discarding rasterized samples having a corresponding stencil value in the opacity stencil buffer state that indicates respective samples in the rendered image are opaque.
 8. The method of claim 1, further comprising writing a neutral value to each stencil value that is read from the path stencil buffer when determining the surviving pixels.
 9. The method of claim 1, wherein the shading of the surviving pixels comprises blending the surviving pixels using an under blend mode.
 10. The method of claim 1, wherein determining the surviving pixels includes discarding rasterized samples having a corresponding stencil value in the stencil buffer that is equal to a neutral value.
 11. The method of claim 1, further comprising initializing the stencil buffer to indicate that all samples of the opacity stencil buffer state are non-opaque.
 12. The method of claim 1, wherein the shading of the surviving pixels includes updating alpha values for the surviving pixels.
 13. A system for rendering paths in top-to-bottom order, the system comprising: a memory that is configured to store a stencil buffer that includes a path cover stencil buffer state and an opacity stencil buffer state; and a processor that is coupled to the memory and configured to: receive a top path including top path geometry and top path covering geometry; generate, by rendering the top path geometry, the path cover stencil buffer state indicating samples of the top path to be covered; determine, during rendering the top path covering geometry with stencil testing, surviving pixels that are covered by the top path based on the stencil buffer; update the opacity stencil buffer state for the surviving pixels; and shade the surviving pixels to produce a rendered image of the top path.
 14. The system of claim 13, wherein the processor is further configured to update the opacity stencil buffer state for each surviving pixel that is opaque after shading each respective surviving pixel.
 15. A non-transitory computer-readable storage medium storing instructions that, when executed by a processor, cause the processor to render paths in top-to-bottom order, by performing the steps of: receiving a top path including top path geometry and top path covering geometry; generating, by rendering the top path geometry, path cover stencil buffer state indicating samples of the top path to be covered; determining, during rendering the top path covering geometry with stencil testing, surviving pixels that are covered by the top path based on a stencil buffer that includes the path cover stencil buffer state and opacity stencil buffer state; updating the opacity stencil buffer state for the surviving pixels; and shading the surviving pixels to produce a rendered image of the top path.
 16. The non-transitory computer-readable storage medium of claim 15, further comprising: receiving a next path that is below the top path and includes next path geometry and next path covering geometry; generating, by rendering the path geometry of the next path, next path cover stencil buffer state indicating samples of the next path to be covered based on the opacity stencil buffer state.
 17. The non-transitory computer-readable storage medium of claim 16, further comprising: determining, during rendering the next path covering geometry with stencil testing, surviving pixels that are covered by the next path based on the stencil buffer that includes the next path cover stencil buffer state and the opacity stencil buffer state; and shading the surviving pixels to produce a rendered image of the top path and the next path.
 18. The non-transitory computer-readable storage medium of claim 16, wherein the generating of the next path cover stencil buffer state includes discarding samples having corresponding stencil values in the opacity stencil buffer state indicating that respective samples of the rendered image are opaque.
 19. The non-transitory computer-readable storage medium of claim 15, further comprising updating the opacity stencil buffer state for each surviving pixel that is opaque after shading each respective surviving pixel.
 20. The non-transitory computer-readable storage medium of claim 15, wherein the opacity stencil buffer state comprises a most significant bit stored for each sample in the stencil buffer. 